Recently, lithium ion batteries are being widely applied not only for comparatively low power purposes such as in mobile telephones and smartphones, but also for medium power to high power purposes such as electric cars, electric buses, hybrid electric vehicles (HEV), and a backup for fuel cell vehicles. Moreover in 2009 plug-in hybrid vehicles (PEV) came into use, these PEVs employing medium to large size lithium ion batteries. In 2008, large size electric double layer capacitors came into practice as a power source for rapid heating of heating drums in high-speed commercial copiers.
Recently, laminated exterior casing type electrochemical devices as lithium ion battery for HEVs and PEVs and medium size to large size electrochemical devices such as large size electric double layer capacitors are paying attention from said prospect of weight and cost reduction. However, laminate type electrochemical devices are low in strength with respect to internal pressure, and there is a need to provide a safety vent in order to release gas generated inside.
In laminated batteries, safety mechanisms have been proposed wherein a vent body configured from an elastic body such as rubber or a coil spring is provided to an exhaust hole formed in a laminated sheet of an exterior casing, and gas inside said battery is spouted out to the outside when said internal pressure of said battery has risen to a specific level (Japanese Patent Application Laid-Open (JP-A) No. 2007-157678).
Moreover, in for example lithium batteries, structures have been disclosed wherein in order to prevent leakage of an electrolyte at the same time as spouting gas out, small gas venting holes are provided in said exterior casing of a lithium ion battery, and a porous membrane with a continuous air bubble manufactured by stretching a fluororesin (PTFE) film is disposed in said small holes (JP-A No. H5-159765).
However, in a safety vent having a configuration wherein a vent body is provided over an exhaust hole formed through a laminated sheet of an exterior casing, there are issues with said battery long cycle life decreasing as a result of electrolyte leaking out while the vent is open unless any measure is taken for preventing electrolyte leakage during vent release in the case of safety vents in which a vent is opened to spout out internal gas when the internal pressure of said electrochemical device has reached a target pressure or above.
In structures in which a porous fluororesin film is disposed in small gas venting holes provided in an exterior casing, there are issues with said production yield of said porous film, as well as the possibility of not only gas, but also electrolyte, being expelled at the same time as gas release in normal use, causing a deterioration in said battery characteristics.